Many schemes exist for improving the power factor (PF) of an electrical appliance, specifically a lighting device. In most cases the increase in power factor is found at the expense of efficiency, cost, complexity or board space. Many a lighting designer has come up with a clever way to improve the PF of a lighting device only to have its efficiency degrade once a PF correction scheme is instituted.
With reference to FIGS. 1A and 1B, the typical high efficiency lighting, using light emitting diode (LED) bulbs 14 as the light source, is often driven by a constant current source module 12 that receives power from a diode bridge rectifier 10 with the filter capacitor C that smooths out the voltage waveform. FIG. 1B illustrates the relationship of input current and voltage, indicating that the circuit of FIG. 1A has low PF and high harmonic distortion. The filter capacitor C discharges slowly but charges very quickly. It results in sharp current spikes 140 in every half cycle of the input voltage 101. The PF value for FIG. 1B is 0.5.
Bridge rectifier 10 and filter capacitor C rectify an incoming alternating (AC) voltage, supplying a pulsating DC voltage for the current source module 12, as is commonly used in many electronic devices. As shown in FIGS. 2A and 2B, a current limiting device (CLD) 16 can be inserted in series with the filter capacitor C for improving the power factor. FIG. 2B shows the simulated results of the input current 100 and the input voltage 101 after the addition of a CLD device. The filter capacitor's C current is limited to some finite value during each half cycle of the input voltage 101. The PF of the circuit simulated in FIG. 2B is 0.8.
Another benefit of the CLD device is that the inrush current into the electronic device is naturally limited to a modest amount which eliminates the need for an inrush current protecting device.
However, in order to limit the current, a voltage 161, as shown in FIG. 2B, must be developed across the CLD device. The CLD sustains a significant voltage 161 and current while it is limiting current (inrush current or capacitor's current), which results in wasted energy in the form of heat. Therefore, the drawback of the additional CLD device is a loss of efficiency compared to the case without a CLD device